System and method for acquiring data

ABSTRACT

A system for acquiring, and displaying, data such as physiological data, from a plurality of data connection devices, each of which monitor one or more different parameters and output data at different sampling frequencies based on their own system clocks. The system receives the data signals at different sampling frequencies and associates each sample of each signal with a time stamp derived from a single master clock. Low rate and high rate data are treated differently. Low rate data is associated with the current value of the master clock, where as high rate data is time stamped by giving the first sample a time stamp equal to the current value of the current master clock, subsequent samples being given an estimated time stamp based on the expected interval between samples derived from the sampling frequency of the data collection device, and the timescale given to the first example. The estimated time stamp may be periodically corrected, and the estimation calculation can be improved by correcting the value used for the interval between samples. The different signals can be displayed together on a display aligned with respect to a time axis. The system can display, the data in two different timescales, one showing a few seconds of data and one showing a few hours of data. The data traces are scrolled across the time axis, new data being added to one end of the trace.

This application is the US national phase of international applicationPCT/GB01/02549 filed Jun. 8, 2001, with designated the US.

TECHNICAL FIELD

This invention relates to the acquisition of data from a plurality ofdata collection devices each monitoring one or more differentparameters, and in particular to the synchronisation of that data.

BACKGROUND AND SUMMARY

There are many different situations in which a system is monitored by aplurality of sensors. Often such sensors are part of different datacollection devices, and monitor the same or different parameters of thesystem. In this situation each of the data collection devices mayinclude its own clock controlling the sampling of the data signal it ismonitoring. These clocks may be free running with respect to each other.Thus the output signals from the devices may not be synchronised and maybe at widely differing rates.

For instance, it is normal when monitoring the condition of a patient tomonitor a variety of physiological parameters such as theelectrocardiogram (which can be multiple channel), blood pressure,respiration, oxygen saturation using pulse oximetry and temperature.Typically these are acquired by different data collection devices andall are acquired at different sampling rates. For exampleelectrocardiograms (ECG) are typically collected at 256 Hz, pulseoximetry waveforms are typically acquired at 81.3 Hz, respirationwaveforms at 64 Hz, temperature at 1 Hz and blood pressure once every 10or 20 minutes. All of these vital signs are of clinical significance andare usually displayed so that medical staff can easily monitor thecondition of the patient. However, because all are measured at differentrates, and typically by different pieces of apparatus with respectivesystem clocks within them, displaying the different parameters togetherin a concise and synchronised way is difficult.

In order to overcome the problem of synchronizing the different signals,one solution has been proposed which is to drive all of the differentmonitors by the same clock signal. However, this requires that all ofthe monitors are, in essence, integrated which is expensive andinflexible, and further this makes existing equipment redundant.

The display of the data is also rendered difficult because parameterssuch as the ECG trace vary on a fast timescale compared to parameterssuch as blood pressure (which is only measured every 10 to 20 minutes).Thus the timing of samples in an ECG trace needs to be accuratelyrecorded. However, the timing of samples of the blood pressure can be oflower accuracy without the loss of clinical significance.

Similar problems arise in other systems, such as plant monitoring andcontrol, e.g. of chemical processing plants, monitoring and control ofmachines, such as engines or vehicle systems.

According to the present invention there is provided a system foracquiring data from a plurality of data collection devices eachmonitoring a parameter and outputting a data signal at a respectivesampling frequency based on respective system clocks, the systemcomprising

-   -   data processing means having:        -   input means for receiving data signals from each of the            plurality of data collection devices;        -   a master clock for providing a master clock signal; and        -   time stamping means for associating a time stamp derived            from the master clock with each of the data signals.

Thus the invention allows data to be collected from a variety ofdifferent data collection devices, but the data samples are given atimestamp which is synchronised with a master clock.

The timestamp may have a higher resolution than the master clock. Themaster clock produces a new time value at regular intervals. The numberof such intervals within a second is known as the tick-rate. Theresolution on the time axis is the inverse of the tick-rate.

Preferably the time stamp associated with the samples is calculated in adifferent way depending on the sampling frequency of the data signal.For data signals (such as in a physiological environment the bloodpressure or temperature) whose sampling frequency is below apredetermined threshold, each sample of the data is associated with atime stamp which is simply the value of the master clock signal at thetime the data is given the timestamp. However for data signals whosesampling frequency is above the predetermined threshold (such as in aphysiological environment the ECG, pulse oximetry or respirationwaveforms) a first sample (or an appropriate sample in a first batch) ofthe data signal is associated with the value of the master clock signalat the time of time stamping, but subsequent samples are provided withan estimated time stamp. This may be based on a time interval calculatedfrom the sampling frequency of the data collection device providing thatsignal (based on the known specifications of the data collectingdevice).

Preferably the estimate is periodically compared with the current valueof the master clock to determine whether the difference between them isacceptable, or greater than a predetermined amount. If it is greaterthan the predetermined amount then the time stamp is corrected. Further,the time stamps of a contiguous set of samples preceding the currentsample are also adjusted, for instance by adjusting them so that theyare evenly spaced in time up to the current sample. The predetermineddifference below which correction is regarded as unnecessary may be amultiple (between 5 and 50, for example) of the master clock'sresolution and the predetermined threshold of sampling frequency may beless than or equal to the master clock frequency, preferably less thanone fifth of the master clock frequency.

As well as adjusting the time stamps of the set of samples preceding thecurrent sample, the manner in which the time stamp is estimated forfuture samples can be adjusted by adjusting the value of samplinginterval used in the calculation. Thus by correcting that value it ishoped that the estimated time stamp will not diverge (or not diverge soquickly) from the value of the master clock. This adjustment can beachieved using a Kalman filter in which the value for the accuracy ofthe sampling interval is set in accordance with the time taken for theestimated time stamp to diverge significantly from the master clock.

In one embodiment for use in monitoring a physiological system (such asa patient) the system is suitable for receiving and displaying signalsfrom an ECG monitor, oxygen saturation monitor, respiration monitor,blood pressure monitor and thermometer, or indeed any other transduceror monitor used for acquiring physiological data.

Preferably the system is based around a data processing device, whichincorporates the master clock, the time stamping means and the display,and the system may be ruggedized so as to be easily portable withoutrisk of damage.

To improve the clarity of the display the data may be displayedselectively on one of two different timescales which may be referred toas a short term continuous timescale, e.g. a “beat-to-beat” timescale ina physiological environment, as in which the time axis shows a shortperiod of data in detail, e.g. a few seconds of data (typically from 1to 60), and a “trend” timescale in which the time axis shows a longersection of data, e.g. a few hours of data (typically this may go from 1minute to 1 day).

The parameters displayed and the manner of their display may be variedbetween the two types of display. For instance, on displaying data atthe first timescale, data sampled at a low sampling frequency, can bedisplayed as a numerical value, rather than a continuous trace (whichwould have little meaning at this timescale given its much lowersampling frequency). On the other hand, in the “trend” timescale it maybe useful still to display a single high frequency trace at the shortertimescale so that a continuous visual check of this trace can bemaintained, even though the rest of the data is viewed over a longtimescale. Preferably key values for the system, such as in thephysiological environment the heart rate, blood pressure, oxygensaturation and temperature, are always displayed as numerical valuesalongside the traces in both display modes.

As a further improvement of the display the representations of thesignals, namely the traces, may be scrolled with respect to the timeaxis as the data signals are received. This contrasts with the normalpractice when displaying signals of refreshing the displayed tracerepeatedly.

The invention provides a corresponding method of synchronizing datasignals and the invention maybe embodied as a computer programcomprising program code means for carrying out the method. The inventionthus extends to a computer-readable storage medium carrying such aprogram.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described by way of non-limitative examplewith reference to the accompanying drawings in which:

FIG. 1 shows an acquisition and display apparatus according to anembodiment of the invention;

FIG. 2 illustrates an example of a “beat-to-beat” display mode of theembodiment of FIG. 1; and

FIG. 3 shows an example of the “trend” display mode of the embodiment ofFIG. 1.

DESCRIPTION OF EXAMPLE EMBODIMENTS

An embodiment of the invention will be described applied to themonitoring of physiological parameters, though the invention can beapplied to any system being monitored by devices running on independentclocks. Typical examples include vehicles, manufacturing or processingplants, control or monitoring systems for instance for the environment.

Referring to FIG. 1 the illustrated embodiment of the invention is basedon a ruggedized laptop computer which includes a display (1) a keyboard(3) and touchpad (5) as data input devices and connectors (7) to whichcan be connected the various types of data collection device from whichphysiological data is to be acquired.

This embodiment of the invention is suitable for receiving signals fromsuch devices as an electrocardiograph, blood pressure monitor,respiration monitor, oxygen saturation monitor and thermometer and, aswill be appreciated from the discussion above, typically all of thesesignals are acquired by those devices at different sampling rates. Thisembodiment of the invention provides for the display, and storage, ofthese parameters synchronised with each other. Thus although each of thedata collection devices has an independent clock which is free-runningrelative to the clocks of the other devices, the invention provides forthe time stamping of each sample of data from each data collectiondevice with respect to a master clock. In this embodiment this is themaster clock of the laptop computer, which has a resolution ofapproximately {fraction (1/18)}th of a second.

The master clock used in this embodiment is sufficiently accurate, buthas a rather low resolution, particularly compared to the timescale ofthe ECG trace. Further, this embodiment is designed to allow for theprocessing of the data in batches. In order to achieve this the incomingdata from the devices is separated into two classes. The first class islow-rate data, which arrives at a frequency of less than {fraction(1/10)}th of the resolution of the master clock, and may actually arriveat irregular intervals. The second class is high-rate data which arrivesat a higher frequency. The higher frequency data is generally generatedat a regular rate.

In this embodiment the data arriving at low rate, such as blood pressuremeasurements or temperature, are time stamped with a sample from themaster clock. In other words, each sample of data is associated with thevalue of the master clock at the time of time stamping of the data.

The high rate data is treated differently. The first sample received (oran appropriate sample in a first batch) is given a time stamp from themaster clock. An expected time interval between samples from the datacollection device providing this physiological data is estimated basedon the specifications of the data collection device. Thus, quite simply,for an ECG where the sampling rate is 256 Hz, the estimated interval is{fraction (1/256)} seconds. Subsequent samples of the data following thefirst are then given an estimated time stamp which is the time stamp forthe previous sample plus the expected time interval.

In order to allow for the estimated time stamp diverging from the masterclock, at regular intervals (e.g. a low multiple of the resolution ofthe master clock typically 5 to 50), the time stamp given to a sample iscompared with the current value of the master clock. In batch processingthis is done on the last value in the batch to give maximum accuracythrough the batch. If the time stamp given to the sample and the masterclock are in close agreement, i.e within a low multiple of the masterclock's resolution, then the process continues. However, if theagreement is insufficient, two procedures are carried out:

-   -   a) The time stamp of that sample is corrected to the current        value from the master clock and a sufficiently long contiguous        set of previous (e.g. covering one second or so) samples have        their time stamp adjusted so that they are evenly spaced up to        the new time stamp given to the current sample.    -   b) Also the value of the time interval used in the estimate of        the time stamps is corrected. Thus rather than using the value        calculated from the specifications of the data collection        device, that value is adjusted to try to achieve lower        divergence from the master clock. The correction can be weighted        according to the accuracy of the clock and the accuracy of the        estimated time interval, for instance using a Kalman filter        cycle in which the accuracy of the expected time interval is        related to the length of time it takes for a timing error to be        deemed to have occurred. However, the adjustment can be made in        different ways.

Thus the incoming data from the data collection devices is synchronizedin software and this avoids the need to drive the different datacollection devices using a single master clock.

The synchronized data is, in this embodiment, stored on a hard disk or a1-gigabyte PCMCIA disk allowing 96 hours of continuous synchronizedpatient data or it can be transmitted to a remote store. Further, thesynchronization of the data allows the signals from the different datacollection devices to be displayed on the single display 1 aligned withrespect to the time axis. Examples of the displays are shown in FIGS. 2and 3. In this embodiment the data may be displayed in two differentmodes, a “beat-to-beat” mode in which five seconds of data are displayedgraphically (see FIG. 2), and a “trend” mode in which five hours of datais displayed (see FIG. 3). The user can switch selectively between thetwo modes, and because all of the data received is time stamped andstored the user can zoom in on key events by switching from the trendmode to the corresponding time point in the beat-to-beat mode. Further,in this embodiment the displays are scrolled along the time axis withnew data being added on the right hand side. This contrasts with atypical display of clinical data in which one trace is generated anddisplayed, e.g from left to right, and then that trace is refreshed,again, from left to right, by new data being overwritten on the oldtrace.

Referring in more detail to FIG. 2, which shows the beat-to-beat mode,it can be seen that three channels of ECG are displayed as traces 21, 22and 23 together with an oxygen saturation trace 24 and a respirationtrace 25. The five traces are aligned vertically one above the other ona time axis showing five seconds of data. In addition important vitalsigns are shown in numerical fashion on the right hand side of thedisplay, these being the heart rate, blood pressure, oxygen saturationand temperature.

This beat-to-beat mode display can be compared with the longer timescaletrend display in FIG. 3. In FIG. 3 the traces 31, 32, 33 and 34 showdata for a five hour period. This timescale can be varied as desired sothat data covering from one minute to one day can be displayed. Trace 31shows the heat rate in beats per minute, trace 32 shows the bloodpressure in millimeters of mercury (and it can be seen that values ofthe systolic and diastolic blood pressure are displayed one above theother, but they only appear every 20 minutes or so), the trace 33 showsthe oxygen saturation and trace 34 shows the temperature. It should alsobe noted that although the display is in the “trend” mode, neverthelessa single channel of ECG trace 35 is shown at the faster timescale (i.e.5 seconds of data over the time axis). This allows proper monitoring ofthe current condition of the patient. The respiration can also bedisplayed as a trace at 36.

In a similar fashion to the beat-to-beat mode display, numerical valuesof the key parameters are shown on the right hand side of the display.

It should be appreciated that the system can be adapted to acquiredifferent physiological parameters from different types of datacollection device, using the same time stamping principles.

1. A system for acquiring data from a plurality of data collectiondevices each monitoring one or more parameters and outputting a datasignal at a respective sampling frequency based on respective systemclocks, the system comprising data processing means including: inputmeans for receiving the data signals from each of the plurality of datacollection devices; a master clock for providing a master clock signal;and time stamping means for associating a time stamp derived from themaster clock with each of the data signals, wherein for data signalswhose sampling frequency is below a predetermined threshold, the timestamping means associates as the time stamp a sample of the master clocksignal, and for data signals whose sampling frequency is above thepredetermined threshold, the time stamping means associates as aninitial time stamp for a sample of the data signal a sample of themaster clock signal and, for subsequent samples of the data signal,estimated time stamps.
 2. A system according to claim 1, farthercomprising: a display for displaying a representation of the datasignals aligned with respect to a time axis on the basis of saidrespective time stamps.
 3. A system according to claim 1, wherein thetime stamp has a higher resolution than the master clock.
 4. A systemaccording to claim 1, wherein the estimated time stamps are based on atime interval calculated from the sampling frequency of the datacollection device providing the data signal and the initial time stamp.5. A system according to claim 1, wherein the predetermined threshold isn/m of the master clock frequency, where m is a positive integer and nis a non-zero positive integer less than m.
 6. A method according toclaim 5, wherein the predetermined threshold is {fraction (1/10)} of themaster clock frequency.
 7. A system according to claim 1, which isoperable with data collection devices whose system clocks arefree-running with respect to the master clock.
 8. A system according toclaim 1, further comprising a data storage device for storing the datasamples and time stamps.
 9. A system according to claim 1, wherein thesignals are physiological signals.
 10. A system according to claim 1,wherein the input means comprises interfaces for receiving signals fromat least two of: an ecg monitor, oxygen saturation monitor, respirationmonitor, blood pressure monitor, thermometer, intra-cranial pressuremonitor, partial oxygen pressure monitor and partial carbon dioxidepressure monitor.
 11. A system according to claim 1, wherein thedisplayed representations of the data signals are scrolled with respectto the time axis as the data signals are received.
 12. A systemaccording to claim 1, wherein the data processing means comprises acomparator, and wherein, for the data signals whose sampling frequencyis above the predetermined threshold, the comparator periodicallycompares the estimated time stamp being associated with the currentsample with the master clock to determine the difference between themand, if the difference is greater than a predetermined amount, the dataprocessing means corrects the estimated time stamp to correspond to themaster clock signal.
 13. A system according to claim 12, wherein, if thedifference is greater than the predetermined amount, the data processingmeans adjusts the time stamps of a contiguous set of samples precedingthe current sample.
 14. A system according to claim 13, wherein the dataprocessing means adjusts the time stamps of the contiguous set ofsamples preceding the current sample by an equal fraction of thedifference such that they are evenly spaced in time up to the currentsample.
 15. A system according to claim 13, wherein the number ofsamples in the contiguous set is substantially the number of samplesacquired in one second.
 16. A system according to claim 13, wherein theestimated time stamps are based on a time interval calculated from thesampling frequency of the data collection device providing the datasignal and the initial time stamp, and, if the difference is greaterthan predetermined amount, the data processing means produces theestimated time stamps for subsequent samples based on an adjusted valueof the time interval.
 17. A system according to claim 16, wherein thedata processing means comprises a Kalman filter for calculating theadjusted value of the time interval.
 18. A system according to claim 16,wherein the data processing means comprises calculating means forcalculating the value of the accuracy of the time interval in accordancewith the time taken for the predetermined amount to be reached.
 19. Asystem according to claim 2, comprising a laptop computer incorporatingthe master clock, the time stamping means and the display.
 20. A systemaccording to claim 2, wherein the data processing means and the displayare adapted to display a representation of the data signals selectivelyon one of two different timescales.
 21. A system according to claim 20,wherein the two different timescales are a first timescale of a fewseconds to a few minutes and a second timescale of one minute to a fewdays.
 22. A system according to claim 21, wherein the first timescale issuch that less than thirty seconds of the time axis is displayed.
 23. Asystem according to claim 21, wherein the first timescale is such thatless than ten seconds of the time axis is displayed.
 24. A systemaccording to claim 21, wherein on displaying the representation of thedata at the first timescale, data sampled at a low sampling frequency isdisplayed as a numeric value.
 25. A system according to claim 21,wherein the second timescale is such that more than one hour of the timeaxis is displayed.
 26. A system according to claim 21, wherein thesecond timescale is such that five or more hours of the time axis isdisplayed.
 27. A system according to claim 21, wherein on displaying therepresentation of the data at the second timescale, one trace of datasampled at a high sampling frequency is simultaneously displayed at thefirst timescale.
 28. A system according to claim 27, wherein the onetrace of data is an ecg trace.
 29. A method of synchronising datasignals from a plurality of data collection devices, each monitoring aparameter and outputting a data signal at a respective samplingfrequency based on respective system clocks, the method comprisingassociating a time stamp derived from a master clock with each of thedata signals, wherein associating the time stamp comprises: for datasignals whose sampling frequency is below a predetermined threshold,associating as the time stamp a sample of the master clock signal; andfor data signals whose sampling frequency is above the predeterminedthreshold, associating as an initial time stamp for a sample of the datasignal a sample of the master clock signal, and, for subsequent samplesof the data signal, estimated time stamps.
 30. A method according toclaim 29, wherein the time stamp has a higher resolution than the masterclock.
 31. A method according to claim 29, wherein the estimated timestamps are based on a time interval calculated from the samplingfrequency of the data collection device providing the data signal andthe initial time stamp.
 32. A method according to claim 29, wherein thepredetermined threshold is less than or equal to the master clockfrequency.
 33. A method according to claim 32, wherein the estimatedtime stamps are based on a time interval calculated from the samplingfrequency of the data collection device providing the data signal andthe initial time stamp and, if the difference is greater than apredetermined amount, the estimated time stamp for subsequent samples isbased on an adjusted value of the time interval.
 34. A method accordingto claim 33, comprising a Kalman filtering process for calculating theadjusted value of the time interval.
 35. A method according to claim 33,wherein the value of the accuracy of the time interval is calculated inaccordance with the time taken for the predetermined amount to bereached.
 36. A method according to claim 29, wherein, for the datasignals whose sampling frequency is above the predetermined threshold,the estimated time stamp being associated with the current sample isperiodically compared with the master clock signal to determine thedifference between them and, if the difference is greater than apredetermined amount, the estimated time stamp is corrected tocorrespond to the master clock signal.
 37. A method according to claim36, wherein, if the difference is greater than the predetermined amount,the time stamps of a contiguous set of samples preceding the currentsample are adjusted.
 38. A method according to claim 37, wherein thetime stamps of the contiguous set of samples preceding the currentsample are adjusted by an equal fraction of the difference such thatthey are evenly spaced in time up to the current sample.
 39. A methodaccording to claim 37, wherein the number of samples in the contiguousset is substantially the number of samples acquired in one second.
 40. Amethod according to claim 39, wherein the signals are physiologicalsignals.
 41. A method according to claim 29, wherein the signalscomprise signals from at least two of: an ecg monitor, oxygen saturationmonitor, respiration monitor, blood pressure monitor, thermometer,intra-cranial pressure monitor, partial oxygen pressure monitor andpartial carbon dioxide pressure monitor.
 42. A computer-readable mediumfor storing executable instructions for performing the method of claim29.
 43. A system for acquiring data from a plurality of data collectiondevices each monitoring one or more parameters and outputting a datasignal at a respective sampling frequency based on respective systemclocks, the system comprising: inputs receiving the data signals fromeach of the plurality of data collection devices; a master clockproviding a master clock signal; and a time stamping device forassociating a time stamp derived from the master clock with each of thedata signals, wherein for data signals whose sampling frequency is belowa predetermined threshold, the time stamping device associates as thetime stamp a sample of the master clock signal, and for data signalswhose sampling frequency is above the predetermined threshold, the timestamping device associates as an initial time stamp for a sample of thedata signal a sample of the master clock signal and, for subsequentsamples of the data signal, estimated time stamps.
 44. A systemaccording to claim 43, wherein each estimated time stamp is determinedby adding an estimated time interval derived from the sampling frequencyof the data collection device providing the data signal to a previoustime stamp.
 45. A system according to claim 43, wherein the inputs, themaster clock and the time stamping device are embodied as a computerdevice.